In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifice for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The nozzle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The nozzle valve is positioned in a nozzle cavity and biased by a nozzle spring to block fuel flow through the nozzle orifices. In many fuel systems, when the pressure of the fuel within the nozzle cavity exceeds the biasing force of the nozzle spring, the nozzle valve element moves outwardly to allow fuel to pass through the nozzle ofifices, thus marking the beginning of injection.
In another type of system, such as disclosed in U.S. Pat. No. 5,819,704, the beginning of injection is controlled by a servo-controlled nozzle valve element. The assembly includes a control volume positioned adjacent to the outer end of the nozzle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the nozzle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the nozzle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the nozzle valve. U.S. Pat. No. 5,860,597 to Tarr discloses a similar servo-controlled nozzle assembly for a fuel injector which controls drain flow through the drain circuit by positioning of the control valve element relative to the valve.
The above-described servo-controlled nozzle valve injectors also function effectively at minimizing lost energy by eliminating nozzle valve guide and control piston leakage paths to drain. This advantage is achieved by using a pressurized spring cavity typically fluidically connected to the nozzle valve cavity. As a result of using a pressurized spring cavity, the nozzle valve element and the control piston must be connected or formed integrally to permit operation. Consequently, there is a need for precise alignment between the nozzle valve element guide surfaces/seat and the control piston bore. FIG. 1 illustrates a conventional servo-controlled injector 10 including an injection control valve 12 for controlling the timing and metering of injection by controlling the drain flow of fuel from a control chamber 14. The injector 10 further includes a nozzle valve element 16 and a control piston 18 formed integrally. A precision ring dowel 20 is used to align the nozzle housing 22 forming the nozzle valve element guide surfaces 24 and seat 26, with the control piston bore 28 formed by the injector body 30. However, even with costly precision machining, this design does not sufficiently align the various guide surfaces of the nozzle housing and the control piston bore. This misalignment undesirably causes the nozzle valve element to experience excessive friction, wear and possibly even stiction. In addition, disadvantageously, a non-symmetrical spray pattern may result in valve covered orifice type nozzle valves thereby adversely affecting combustion.
U.S. Pat. No. 4,798,186 discloses a nozzle controlled fuel injector including a control volume formed by a piece positioned on the outer end of the nozzle valve element. The piece is not guided at its outer circumference to provide a substantially leak free, seal-tight design, and an unhindered axial motion of the injector nozzle valve thereby reducing undesired frictional forces which would occur in case all tight fits needed for a tight seal were not perfectly concentrical to one another. The piece completely encircles the outer end of the nozzle valve element. The piece is biased into engagement against a flat lower surface of a support only by fuel pressure in the control volume. When the control valve opens and drains fuel from the control volume thereby decreasing control volume pressure, the bias force holding the piece against the flat surface of the support decreases substantially. When the control valve is closed, the fuel pressure induced biasing forces against the piece are substantial thereby preventing lateral movement of the piece. Also, two annular leakage passages (63, 69) separate the high pressure regions in the spring cavity and the control chamber. At high pressures, this leakage to drain can be significant.
U.S. Pat. No. 5,067,658 discloses a nozzle controlled injector including a control chamber positioned at the outer end of a nozzle plunger and a top sleeve for slidably receiving the outer end of the plunger. The outer end of the sleeve functions to seal the control chamber. In addition, an elastic element is used to fixedly position the top sleeve by providing a slack free assembly. However, the top sleeve is positioned in an axial bore with a sufficiently close tolerance fit to require an axial groove in the sleeve to permit passage of fuel flow through the close tolerance fit. Therefore, this design does not provide sufficient lateral movement of the top sleeve to accommodate for misalignments.
U.S. Pat. No. 4,605,166 is noted for disclosing an injector including a nozzle valve element and a sleeve mounted on an outer end of the nozzle valve element wherein the sleeve sealingly engages a support while being biased into engagement by a bias spring which also biases the nozzle valve element into a closed position. However, the collar or sleeve functions as a check valve and is therefore axially movable.
Therefore, there is a need for a simple, inexpensive nozzle controlled injector assembly capable of effectively permitting precise alignment of nozzle valve guiding surfaces and a control piston bore while permitting effective control of fuel metering.